SO WHAT IS ELASTIN?
Some Questions and Answers
by
*Howard M. Lenhoff, Executive Vice
President
Williams Syndrome Foundation, and
Professor of Developmental and Cell Biology
University of California, Irvine, CA 92697-2300
A gene for elastin is missing: Dr. Colleen
Morris of the University of Nevada and her coworkers at the
University of Utah discovered that all individuals with Williams
syndrome tested lacked a small piece of one of their two chromosomes
called
Chromosome # 7 [NOTE: Our chromosomes come in pairs, one from
each parent]. In individuals without WS both of their chromosome #
7s were intact. One of the genes in the missing piece of chromosome
#7 was identified as the gene for making elastin. In the following,
I try to answer some of the questions that you may have regarding
genes, elastin, and the consequences of this discovery for your
children.
***
How do most genes work? Most genes are responsible for the synthesis
of a specific protein. Thus, for each kind of protein in our body,
we have a specific gene responsible for its synthesis.
***
What is a protein? Our bodies can make about 100,000 different kinds
of proteins, each one having a special role to play. All proteins in
our bodies are made up of small molecules, called amino acids,
linked together in coiled chains. There are two general classes of
proteins. Most are functional proteins which make the chemical
reactions in our bodies go very quickly. One example is ptyalin, the
protein in our saliva that digests starch into sugar. The other
proteins, the structural proteins, help organize the structure of
our tissues and organs, and give them strength and flexibility.
Some of these structural proteins are long and fibrous. The most common fibrous protein is collagen. This tough protein is found in our tendons, ligaments, and in the connective tissue of our skin, blood vessels, lungs, and other tissues and organs. The rope-like collagen fibers gives those tissues and organs the rigidity they require to function.
***
What is elastin and where is it found? Elastin is our body's
structural protein that gives elasticity to our tissues and organs.
Elastin is found predominantly in the walls of our arteries, in our
lungs, intestines, and skin, as well as in other elastic tissues. It
functions in connective tissue in partnership with collagen. Whereas
collagen provides rigidity, elastin is the protein which allows the
connective tissues in our blood vessels and heart tissues, for
example, to stretch and then recoil to their original positions.
Imagine elastin within the body's connective tissue to act like a
bunch of rubber bands that are tied together at a number of places.
When the elastic bands are pulled, they will stretch, and when there
is no longer a pull, they will return to their original relaxed
state. You can't pull the elastin chain too far because the
companion stiff collagen fibers in the connective tissue limit the
stretching of the elastin fibers in the tissue.
***
Why is elastin a particularly unusual protein? Elastin is considered
by scientists to be a very tough and relatively stable protein
because it has many internal linkages. Those linkages make elastin
resistant to the normal breakdown characteristic of most proteins.
***
Since elastin is relatively stable, do we need to make elastin
throughout our lives? No! Normally the body stops making elastin
once the body reaches maturity soon after puberty. A geneticist
would say the same thing by stating that "the gene for elastin is
turned off just after puberty." In other words, once the body has
made its elastin, it will not make that protein any more.
***
What is the consequence of not being able to make any more elastin
after we mature? In two words, aging begins.
***
So what is the significance of all this to our child with Williams
syndrome? On the one hand it helps us understand why our WS children
have some complications with organs requiring elasticity, such as
arteries and the intestines. On the other hand, at this time we do
not know many of the consequences of the absence of the gene. For
example, although we know that people without WS possess two doses
of the gene for elastin and that people with WS have only one dose,
we do not know if the gene from those with WS may work overtime to
compensate. Furthermore we do not know how the absence of the one
elastin gene affects the structure of the arteries, lung, and other
tissues and organs of the body. That is precisely why further
research is needed.
What is the difference between the elastin made by individuals with
WS and individuals who have inherited supravalvular arterial
stenosis (SVAS)? Unfortunately, we can not say for certain until
further research is done. The following statements, however, reflect
the current thinking of research scientists: Those with WS produce
elastin that has a normal structure, but probably not enough of it.
On the other hand, individuals who have inherited SVAS, have a
mutation of the gene for elastin in one of their chromosomes;
therefore for every molecule of normal elastin they produce, they
also produce one of abnormal elastin thereby giving their "hybrid
elastin" an abnormal structure which leads to a defective elastin
protein.
***
Will the discovery of the missing elastin gene, and the anticipated
future discovery of the properties of elastin in individuals with WS
explain all of the symptoms of WS? Probably not. Remember, a piece
of Chromosome # 7 is missing, and the size of the missing piece
(called a microdeletion) may vary among individuals. We still do not
know the identity of other genes which may also be missing. Perhaps
when we identify the other missing genes, we may be in a better
position to understand more of the symptoms of WS and gain insight
into how to counteract some of the negative aspects of the syndrome.
***
I'm confused; although we know that an elastin gene is missing, are
we right back at square one? WRONG! Dr. Morris and her colleagues
have shown that one chromosome, chromosome # 7, is involved, and
that one specific location on that chromosome, the site of the
elastin gene, is important. Now that scientists have the elastin
gene as a landmark, they can start looking on either side of the
elastin gene for other genes that may affect WS. The needle in the
haystack of 100,000 genes has been found! Now we must find out what
genes are on both sides of that needle.